Auto merge of #97575 - nnethercote:lazify-SourceFile-lines, r=Mark-Simulacrum

Lazify `SourceFile::lines`.

`SourceFile::lines` is a big part of metadata. It's stored in a compressed form
(a difference list) to save disk space. Decoding it is a big fraction of
compile time for very small crates/programs.

This commit introduces a new type `SourceFileLines` which has a `Lines`
form and a `Diffs` form. The latter is used when the metadata is first
read, and it is only decoded into the `Lines` form when line data is
actually needed. This avoids the decoding cost for many files,
especially in `std`. It's a performance win of up to 15% for tiny
crates/programs where metadata decoding is a high part of compilation
costs.

A `RefCell` is needed because the methods that access lines data (which can
trigger decoding) take `&self` rather than `&mut self`. To allow for this,
`SourceFile::lines` now takes a `FnMut` that operates on the lines slice rather
than returning the lines slice.

r? `@Mark-Simulacrum`

Author: bors

compiler/rustc_query_impl/src/on_disk_cache.rs

@@ -713,7 +713,7 @@ impl<'a, 'tcx> Decodable<CacheDecoder<'a, 'tcx>> for Span {
         let len = BytePos::decode(decoder);
 
         let file_lo = decoder.file_index_to_file(file_lo_index);
-        let lo = file_lo.lines[line_lo - 1] + col_lo;
+        let lo = file_lo.lines(|lines| lines[line_lo - 1] + col_lo);
         let hi = lo + len;
 
         Span::new(lo, hi, ctxt, parent)

compiler/rustc_query_system/src/ich/impls_syntax.rs

@@ -69,7 +69,7 @@ impl<'a> HashStable<StableHashingContext<'a>> for SourceFile {
             external_src: _,
             start_pos,
             end_pos: _,
-            ref lines,
+            lines: _,
             ref multibyte_chars,
             ref non_narrow_chars,
             ref normalized_pos,
@@ -79,11 +79,15 @@ impl<'a> HashStable<StableHashingContext<'a>> for SourceFile {
 
         src_hash.hash_stable(hcx, hasher);
 
-        // We only hash the relative position within this source_file
-        lines.len().hash_stable(hcx, hasher);
-        for &line in lines.iter() {
-            stable_byte_pos(line, start_pos).hash_stable(hcx, hasher);
-        }
+        // We are always in `Lines` form by the time we reach here.
+        assert!(self.lines.borrow().is_lines());
+        self.lines(|lines| {
+            // We only hash the relative position within this source_file
+            lines.len().hash_stable(hcx, hasher);
+            for &line in lines.iter() {
+                stable_byte_pos(line, start_pos).hash_stable(hcx, hasher);
+            }
+        });
 
         // We only hash the relative position within this source_file
         multibyte_chars.len().hash_stable(hcx, hasher);